For aircraft utilizing gas turbine engines, such as turbofan engines and the like, air exiting a fan of the engine may flow from the fan into the compressor of the engine. That air is then compressed and directed to a combustor of the engine where it is combined with fuel and ignited. The product of that combustion is then directed to a turbine section of the engine to cause rotation of same. As the turbine section is mounted on the same shaft or shafts as the compressor and fan, all rotate and the process is continued.
However, some of such air exiting the fan may bypass or go around the compressor. For the purposes of the discussion herein, this fan flow stream that bypasses the engine will be referred to as “bypass air.” Moreover, the percentage of air bypassing the compressor relative to the percentage of air entering the compressor is referred to as the “bypass ratio.”
In addition, a portion of this fan air may be captured by an inlet port in a scoop frame generally disposed on the nacelle of the engine and may flow from the inlet of the scoop through one or more ducts or valves to apparatus such as a precooler, heat exchanger, or the like for further use. For example, the captured fan air may be used by the Environmental Control Air Conditioning System (ECS) for cooling of bleed air from the engine for circulation in the aircraft cabin. It may also be used by the Wing Anti-icing System (WAIS) for de-icing of the aircraft wings. Other uses may include cooling of engine oil, fuel, or the like by a heat exchanger located downstream of the scoop frame capturing the extracted fan air.
The fan air ideally flows around a nacelle upper bifurcation in a smooth, streamlined manner to yield low pressure loss in the fan duct stream. The inlet scoop frame may be located at the leading edge of the nacelle upper bifurcation in an attempt to capture the fan air with the highest possible fan air pressure. The inlet scoop frame typically has a single frontal opening generally sized to intake fan air at the maximum fan flow extraction condition, while still allowing a smooth, streamlined fan air flow around the nacelle upper bifurcation. This maximum fan flow extraction condition, typically considered the design point for the inlet scoop, is often associated with aircraft icing conditions at a specific aircraft holding altitude, flight speed and engine power setting.
On the other hand, the aircraft flight at non-icing conditions, and other flight altitudes, flight speeds and engine power settings such as take-off, idle, and climb, are normally considered as off-design points requiring less fan air extraction. Such off-design points may require fan air extraction that is not only less than that at the design point but that may also range down to near zero flow rate at the aircraft takeoff phase. At these off-design points, when the intake of the fan air into the inlet port of the scoop frame is relatively low, flow separation and spillage at the inlet scoop may occur and may result in high turbulence to the fan air stream. Consequently pressure loss to the fan stream at the nacelle nozzle exit may result.
In light of the above, it can be seen that nacelle fan air stream performance penalties may occur under various fan air flow extraction rates because of the flow separation and disturbances to the fan air flow stream. Nacelle fan airstream performance penalties are equivalent to increases in the aircraft fuel burn at cruise and a decrease in the available aircraft takeoff thrust. A better design is therefore needed for the scoop frame to minimize flow separation and spillage at off-design points.